Passive Solar Energy
An overview
Direct solar energy is there for the taking in all its abundance and yet we are only just waking to the fact. The energy crisis of the 70s and the polluting effects throughout the 80s and 90s has caused the world to think seriously about alternative non polluting energy sources. Our free ride on mother Earth, taking its long term storage of solar energy in the form of fossil fuels, may soon be over. And so it is now that we all should be seriously thinking of the alternatives. So if man can devise and adopt a technology to use solar energy in real time which means getting the energy where we want it at the time of asking or even over the short term converting it efficiently to other forms through biotogical growth, we are well on the way to what the Greens call sustainability.
Saving energy by using it more efficiently is actually cheaper than creating the extra supply, in other words it is actually cheaper to save 1 Walt of power than it is to make it available say from a power station. And that is where solar energy can be used passively to make the savings on electricity consumption, be it for industrial or domestic use.
Despite advances in recent years, solar collection and conversion is still in its infancy and falls into two basic types: Active collectors will convert the sun's rays directly into another form of energy, say electricity. The general name for these devices are called transducers. The collection of solar heat in water-piped solar panels is also loosely termed an 'active' system but should not be confused with a transducer. Passive types of solar collector are where we try to store the heat and use the light directly from the sun, the most obvious example being a carefully designed house.
Passive Solar Collecting
Passive solar energy collection, in contrast, has been found to be cost effective and is most economically attractive. If adopted on a wide scale through a process of thoughtful house design, it could make significant savings on the energy consumption of a typical household. Well planned house design for effective solar collection of heat and light would add little or no extra cost to the building.
Firstly, the general sighting and orientation of the house is important. Maximum collection of the suns rays in the northern hemisphere is for the main window area of the house to be southfacing, with minimum window area on the northern side. This is to take advantage of the real and useable greenhouse effect, trapping the lower grade energy infra red waves within the building.
It should go without saying, that all standard house windows should at least be double glazed, trapping the insulating layer of air between the glass to prevent heat escape. Lalest developments in glass technology has reduced the radient heat escape still further by depositing metallic or compound molecular layers on the inside surface of the glass, but it remains to be seen how long it will take for this `high-tech' glass to become affordable and standard.
The next and most fashionable way to add solar heating is to integrate a highly glazed unheated area, more commonly known as a conservalory, to the south side of the building. The conservalory produces two effects. It minimises heat loss from the house wall and secondly the greenhouse heating effect can be used to store the heat within the fabric of the building over a longer period. This evens out the temperature fluctualions inside the building.
The heat can be taken away from a heated wallspace by convector vents al the top and bottom of the wall during the second half of the day. It must be closed at night to prevent a reverse cycle coming into operation.
If space is at a premium, then an alternative to a conservalory is the use of a Trombe wall. Designed by Felix Trombe in 1967, the double-glazed thermal wall is made of a thick, black painted concrete wall. The absorbed sunlight and heat is radiated and convected by vents throughout the room. The Trombe house shows that 70% of its annual heating requirements are supplied by solar energy.
Water Walls And Roof Ponds
Although unusual, a water wall has many advantages over a masonary thermal wall for storing heat. A masonary wall will transfer heat by conduction and takes a long time to heat up owing to its low thermal conductivity, whereas water will transfer heat throughout itself principally by convection. Water stirs itself by this method and heats up more uniformly using all of its mass for storage. It should also be emphasised that water is a poor conductor of heat and in certain circumstances will produce hot spots (when swimming in the sea, the body feels these hot spots), but if the container is taller than it is wide, this is not much of a problem.
Another advantage over concrete is that water will store over twice as much energy for the same volume and for the same temperature rise. The effect of this in broad terms means that inside temperature fluctualions are reduced owing to a larger energy store. Combining a water wall or a material with high heat capacity with a conservalory would optimise conditions on the south facing side of the house. It is at this point, building designers might consider an alternative to brick, having the same strength and durability but with a higher specific heat capacity.
Ralher than having the dubious task of filling the cavity wall with large plastic bags full with water and hoping the seat will not break, is to use. drums or cylindrical cannisters painted black. The 'tin-can' idea might be adopted in a greenhouse to reduce temperature changes from day to night and thus save on heating bills. A matt black surface will absorb 95% of solar radiation and surprisingly, blue follows a close second at around 90%.
A roof pond can provide some backup heating in winter and cooling in summer. Sunlight is collected during the day in winter to heat the water. At night when the covers are over the water, heat escapes through the ceiling by radiation. In summer the reverse effect can be adopted, keeping the white insulating covers over by day, the cool water will absorb heat from the room. The water then releases its energy to the cool almosphere at night with the covers removed.
An essential design consideration for a roof pond is the strength of the ceiling and the water seat. Ribbed steel could be used as it serves the purposes of strength and an increased surface area for re-radiation of heat into the building. Again it would be advisable to segment and contain the water in black vessels or bags to minimise the problems of leaks and to ensure maximum absorption from the sun.
Open water not only has the disadvantage of evaporation and deposition from rain but suffers from an unacceptably high reflectivity, what physicists might call an impedance mismalch. A thin black layer over the surface of the water should go some way to compensale for this.
Clerestory Lights And Atria
Clerestory windows are vertical windows in roof lines and provide two functions. They provide much needed light for interiors of buildings and also passive solar heat. Much to be gained from the winter sun, the clerestory is an under-used component in a typical structure. The winter sun will provide the light for the room and the heat within a rear storage wall. A shading lip over the window minimises entry of the midday summer sun and is adjusted to give maximum sun coverage over the rear wall.
Multiple clerestories will gain an extra 9% of collected energy from the sun in roof structures by using a sawtooth arrangement. The initial reflected energy is captured by the adjoining glass.
Simple shading of windows for the noonday summer sun can be incorporated into a building. A horizontal profection over the window will allow winter sunlight to enter the window and curtail some of the maximum heat in the peak of summer.Traditional shutters over windows could be utilised to reflect sunlight into the house. The same goes for reflective covers over roof lights. A finer point when designing houses, can be through stoping the wall or lintel above a window. This increases the available sunlight in winter.
We might also see the rise in popularity of atria in domestic houses, where considerable energy savings can be made in electric lighting. By making central parts of the roof line transparent usually with structures of glass, the inner parts of the building are opened up to receive the extra heat and light. Further innovative research into light-ducting and light-guide, getting the much needed light into difficult central areas and underground, would complete a chapter in saving energy within buildings.
If Britain and other countries are to make serious energy savings in the hope of reducing electric power consumption and thus pollution, then we need to think carefully about incorporating carefully designed solar houses as standard. For the generating companies, it will pay to conserve in every aspect of electricity consumption.
